In a longitudinal study, the impact of menarche (the onset of menstruation) will be evaluated for adolescent girls. Predictions to be tested derive from a model of girls' biopsychosocial development, viz, that (a) the effects of the biological event of menarche, and the timing of this event in girls' lives are, (b) mediated by the biopsychological variables, such as development of secondary sex characteristics, preparation for menstruation, level of ego development, sense of being developmentally on- or off-time, and are evident in (c) three classes of psychosocial functioning--body image, psychological well-being, and self-concept/self-experience. Sixth, seventh, eighth, and ninth grade girls (N=600), attending public schools, will be tested twice, in the fall and spring of a school year. Girls' menarcheal status will be ascertained on both test occasions. Three groups will be discriminated--consistently premenarcheal girls, consistently postmenarcheal girls, girls whose menarcheal status changes from pre- to postmenarcheal. Within this latter group early maturers (sixth graders) will de distinguished. Change in their psychosocial functioning from the first to second testing will be compared with that of their same-age peers whose menarcheal status remained constant. One-wy multivariate analyses of variance and regression analysis will be used to answer questions related to (a) biopsychosocial changes during the junior high school years, (b) psychosocial impact of menarche, (c) variations in impact of menarche for early, average, and late maturers, (d) variables mediating impact of menarche. Today there is meagre information pertaining to normative patterns of adolescent girls' development. This research will be valuable to those treated healthy girls as well as to those treating girls exhibiting developmental disorders related to sexual maturity.